Technical Field
The present disclosure refers to the satellite positioning systems, and particularly refers to the prediction of satellite orbits.
Discussion of the Related Art
As is known, a GNSS (Global Navigation Satellite System) receiver, for example a GPS (Global Positioning System), for calculating its position, receives correctly formatted electromagnetic signals transmitted by a constellation of satellites orbiting around earth. The receiver position is determined by a triangulation process which requires a rather complex procedure.
Initially, the GPS receiver, by a process known as acquisition, needs to locate a sufficient number of satellites and “hook” to them. However, the GPS receiver for calculating its correct position, needs to know where each of these satellites is positioned in the space, with a high degree of precision.
Each satellite spends 30 seconds for transmitting its navigation message containing its accurate position, and the GPS receiver downloads from each satellite such data, required by the receiver, for calculating the “fix” (that is the position of the receiver itself). These data, known as ephemerides, describe a limited orbit arc and are generally valid only for 4 hours in case of the GPS.
If the reception of this data is interrupted, for example due to a building or tree, the receiver needs to wait additional time to download the missing data from the satellite.
In real conditions, wherein the GPS receiver is often in motion, some minutes are required for the receiver to obtain the availability of all the needed data for making its calculations and obtaining the “fix”, this fact causes an extreme uncertainty about the position before the navigation starts.
A possibility of alleviating the above mentioned problem consists of autonomously generating, inside the receiver, the satellites orbit prediction based on previous ephemerides transferred and stored in the receiver memory. It is known that satellite orbit prediction methods are available for GPS and GNSS systems. Those methods can predict such orbits up to several days.
The autonomous prediction of a satellite orbit is becoming an important characteristic of a GNSS receiver. The orbit of each satellite can be estimated by solving a standard problem of celestial mechanics by a suitable degree of knowledge of all the forces (gravitational and non-gravitational forces) acting on the satellite and the so-called Initial Condition, that is the position of a satellite and its speed at a given time instant which are available from present and/or past observations.
Document U.S. Pat. No. 8,228,230 describes a satellite orbit prediction method.
Among the non-gravitational forces acting on a satellite there is the solar radiation pressure due to the solar wind action on a space vehicle as a satellite.
The effect of the solar radiation pressure on the satellite depends on the shape, size and mass of the satellite itself. Since there are different types of satellites having different parameters of size, shape and mass also in the same constellation, the celestial mechanics model used for predicting their orbits should take in account said parameters for predicting the orbits based on a model of the solar radiation pressure which effectively corresponds to the used satellite.
It is observed that the information regarding the type of a satellite is not a datum transmitted by the satellite itself and therefore cannot be available to the receiver. For example, nowadays the GPS system can use satellites of the GPS Block II-A type, or satellites of the GPS Block II-R type, or satellites of the GPS Block II-F type.
The article “New Empirically derived Solar Radiation Pressure model for global Positioning System Satellites” Y. Bar-Sever and D. Kuang, IPN Progress Report 42-159, pages 1-11, 15 Nov. 2004, describes models of the solar radiation pressure useable for Block II-A or BPS Block II-R satellites.
If the orbit of a satellite is predicted in dependence of a solar radiation pressure model referred to another type of satellite, such prediction is affected by an error influencing the position calculation of the receiving apparatus and therefore the fix calculation.